On occasion, an aircraft may be approaching, entering onto, or in, a closed surface of an airport. Such closed surfaces at an airport may be inadvertently traversed by a landing aircraft or an on-the-ground aircraft. Non-limiting examples of surface areas are runways, gates, ramps, parking stands, taxiways, aprons, or de-icing areas. Surface areas may be closed because of some physical obstruction (‘X’ sign) or a surface issue (such as a milled surface being prepared for fresh concrete, or other construction or maintenance issue). In other situations, the on-the-ground aircraft may attempt to, or turn onto, a runway, taxiway or other surface for which the aircraft is not cleared to be on by the airport controllers. Here, the surface is not physically closed. Rather, the surface is closed to that particular aircraft. In other situations, an airborne aircraft may be attempting to land on a closed runway. For that particular aircraft, airport surfaces for which the aircraft is not cleared to operate is considered a closed surface.
Collisions between vehicles and other objects or other vehicles is a significant problem. Such collisions can frequently be traced to the vehicle inadvertently being driven or piloted into an area that the vehicle is not supposed to be in, at least at that time. The prior art has typically been to mark such areas and rely on the vigilance of the pilot or operator of the vehicle to observe the signage and to not drive the vehicle into the area. This system works most of the time; however, human operators are prone to human error, and the consequences of such collisions, particularly in certain applications, are so catastrophic that additional measures of preventing such accidents are warranted.
As an example, collisions between aircraft on the ground and other vehicles or aircraft are one significant source of accidents in the aircraft transportation system. One potential cause of a particularly catastrophic collision is an aircraft that is on the ground inadvertently taxiing onto a runway where another aircraft is landing or taking off. Another cause of collisions is an aircraft using the wrong runway. Various systems have been adopted in airports, in part to prevent or minimize such runway incursions. Traditional systems for this purpose include requiring permission from an air traffic controller before an aircraft taxis across a runway, watching and monitoring of the movement of aircraft by air traffic controllers, various signage and markings showing aircraft on the ground where to go, and use of aircraft lights while taxiing so that the aircraft can be seen better by air traffic controllers and pilots of other aircraft. However, the adoption of such systems have not eliminated the problem, and runway incursions have increased in recent years. In response to these increases, efforts have been undertaken to increase awareness and improve training of pilots, air traffic controllers, and others in a position to cause or prevent runway incursions. However, traditional technology is not a complete solution as it loses its effectiveness in poor visibility or poor weather conditions, and relies on the repeated and consistent avoidance of human error.
As a result, systems have been proposed, including the Airport Movement Area Safety System (AMASS) to monitor runways and alert air traffic controllers when an aircraft or other large object moves onto a runway. Such systems typically use Airport Surface Detection Equipment (ASDE) radar or other detection equipment to monitor the airport surface areas. Generally, when the system detects a runway incursion, the air traffic controller is alerted, who then must determine which aircraft or other object triggered the alarm, and notify the pilot of that aircraft (if the infringer is an aircraft) that they have traveled into an area where they should not be. Such systems are expensive, complicated, cumbersome, and difficult to use. Many detection systems are typically required for the perimeters of many runway safety areas, and air traffic controllers must either disable the system when an aircraft has clearance to cross a controlled parameter, or must deal with a false alarm each time such a crossing occurs. In addition, when an unauthorized incursion does occur, precious time is lost while the air traffic controller tries to figure out which aircraft (if it was an aircraft) triggered the detection equipment and notifies the pilot of the infraction. Furthermore, the use of different systems at different airports make the pilot's jobs more difficult and increase the likelihood of pilot error.